The purpose of this study is to: 1) compare glycolytic muscle metabolism in patients with chronic fatigue syndrome (CFS) with normal humans, 2) test the feasibility of using near-infrared spectroscopy (NIRS) measurements to evaluate patients with chronic fatigue syndrome, and 3) to determine if exercise training improves muscle metabolism in CFS patients. NIRS equipment is extremely portable and inexpensive. The best use of this methodology is to measure the time constant of oxygen-hemoglobin resaturation after exercise (HbO2 Tc). This measurement represents the oxidative capacity (ability to deliver and use oxygen) of the muscle being tested. To address these aims we will test 20 CFS patients as identified by Dr. Natelson's research group (10 have been currently tested). In addition we will test 10 age, gender, and race matched control subjects (5 have been tested). The subjects will be tested in the MRS facility at the University of Pennsylvania. The subject will lie prone in the magnet and will perform repeated plantar flexions using a isokinetic device. The exercise protocols will consist of 2 maximal exercise bouts (60 seconds in duration) and 2 five minute submaximal exercise bouts. The MRS measuring coil will be placed over the medial gastrocnemius and the ISIS localization method used. The NIRS probe will be placed within the same area. Preliminary studies have shown a good agreement between the MRS and NIRS measurements in control and CFS subjects. In addition the 60 second maximum exercise protocol is well tolerated by the CFS patients. Ten CFS patients are currently being enrolled in a 4 month aerobic exercise training program. These patients are being tested with MRS and NIRS before and after the training program.